My social group is already at the point of needing to be on Facebook, otherwise you miss out on things.
My main social circles, about 30-40 people, ranging in age from early 20s, through to mid 50s, and are all, with one exception, on Facebook.
I watch a lot of live acts, bands, comedians etc. and all the local venues that do this kind of thing are on Facebook, and most of the bands I see regularly are also on there.
So it's very easy to see, in one place, what is going on, when and where, without having to go looking around all over the place, or on individual venues web sites etc. And as Facebook use an Open standard for their Events calendar, this is easily added to things like Google calender, so all the events and details from Facebook automatically show up on my phone. (Android)
This makes organising nights out, letting other people know what's going on etc a breeze.
But this does mean our one friend who won't sign up, gets left out now, as someone has to remember to call or text him every time something new is going on, or details have changed.
I have very little personal info on there, I don't play games, and I don't normally read the friend news feeds etc.